Double-jet precipitation is a common practice in the making of silver halide emulsions. Silver salt solution and halide salt solution are introduced simultaneously, but separately, into the precipitation reactor under mixing. In order to achieve the desired crystal characteristics, typically, the silver ion activity or the halide ion activity is controlled during the precipitation by adjusting the feed rates of the salt solutions using either a silver ion sensor or a halide ion sensor.
Quite often the crystal characteristics change when the process is scaled up or down or transferred to a different reactor. A possible explanation for this change is that silver ion or halide ion activities are not homogeneous throughout the reactor. Thus, although they may be under control at certain locations in the reactor, the concentration profiles are not necessarily reproduced when the reactor is changed. Different concentration profiles of silver ion or halide ion activities in the reactor during precipitation can cause differences in crystal characteristics.
For yield reasons, practical silver halide emulsions are always made by feeding highly concentrated silver salt and halide salt solutions (typically higher than 0.5 moles per liter) to the reactor. The solubility of the silver halide is low, e.g., 10.sup.-6 moles per liter at 70.degree. C. for silver bromide. Thus, in the case of silver bromide emulsions made under conditions of 70.degree. C. and 10.sup.-2 M bromide ion activity, the silver ion and bromide ion activities need to drop from the molar range at the introduction point down to somewhere near 10.sup.-6 and 10.sup.-2 moles per liter respectively in the bulk emulsion. The magnitude of this drop basically guarantees an inhomogeneity in activity of the silver ion and the halide ion.
It is possible that this inhomogeneity in reaction activities can be largely obviated. A hypothetical situation is that if the reactant solutions are instantaneously converted into small nuclei of silver halide at the introduction point, and later redissolved to precipitate onto the existing grains in the bulk solution, the entire drop in reactant activities takes place at the introduction point and the great majority of the reactor can be homogeneous so long as the mixing of the bulk solution is efficient. To what extent this ideal situation is achieved in practical systems depends on the kinetics of nucleation and hydrodynamics at the introduction point. Fast kinetics and effective mixing of the reactants favors the efficient formation of nuclei.
A different view of this problem is to recognize that the inhomogeneity of the reactant activities originates in the introduction of the halide salt and silver salt solutions. When the introduction stops, given efficient bulk mixing, the emulsion is quickly homogenized. Conceptually, if a process is designed in a way such that the time involved in feeding reactant solutions is short compared to that of the entire precipitation reaction, the reactor should be homogeneous most of the time, and an accurate control of reactant activities can be achieved.
In the apparatus disclosed in U.S. Pat. Nos. 4,289,733 and 5,096,690 an approach is taken to better control the hydrodynamics at the introduction point by creating a well-defined primary zone which is separated from the bulk of the reaction vessel. The apparatus and process described in these patents takes the approach of confining the inhomogeneity to a primary mixing zone and hoping that the rest of the reactor will be homogeneous. However, these patents make no attempt to enhance the rate of nucleation. Although the kinetics of nucleation depend somewhat on the silver halide involved, the rate of nucleation is proportional to the level of supersaturation. For a given mixing condition, the higher the feed rate and concentration of the reactants, the higher the supersaturation at the introduction point, and hence the higher the rate of nucleation. As mentioned earlier, when the rate of nucleation is sufficiently high, the inhomogeneity of the reactants will be confined to a small vicinity of the introduction point and this eliminates the need for a physical boundary to define the primary reaction zone described in the above-mentioned patents. Based on this concept, the reactant solution should be introduced at a high flow rate and simultaneously so that when mixed, high supersaturation is achieved to maximize the rate of nucleation.
Another approach suggested in the prior art is the addition of silver salt and halide salt alternately as described in U.S. Pat. No. 4,666,669. However, this process emphasizes the benefit of reactant dilution at the introduction point and, therefore, the rate of nucleation is limited.
The present invention solves the problems of the prior art and provides a double jet process that is highly precise and allows transference from pilot to production scale.